A press of pressure cell type generally comprises a force-absorbing press body which defines a press chamber. In the upper part of the press chamber, a press plate and a diaphragm of rubber or another resilient material are arranged, which together form a pressure cell. The pressure cell communicates with a source of pressure and expands when a pressure medium is supplied. In the lower part of the press chamber, a structural support or a tray is arranged, which comprises a bottom plate having a tray frame. The tray supports a forming tool, a workpiece, a mat of rubber or another resilient material, covering the forming tool and the workpiece.
Presses of pressure cell type are used, among other things, when forming sheet-shaped blanks, for example sheets of steel or aluminium, to short series products within the aircraft industry and the motor industry. The sheet is placed in the press in such a manner that one of its sides faces a forming tool. The resilient diaphragm is arranged on the other side of the sheet. A closed space between the diaphragm and the press plate located above the diaphragm constitutes the pressure cell and this space is filled during the forming process with a pressure medium. By pumping additional pressure medium into the pressure cell, the pressure is increased in the pressure cell and the resilient diaphragm is pressed during stretching against the sheet which, in its turn, is formed round or in the forming tool. When the sheet completely fits to the forming tool, the pressure in the pressure dell is released and the diaphragm is removed, after which the formed component can be taken out of the press.
Another field in which presses of pressure cell type are used is wood compaction when a workpiece of wood is exposed to high pressure, either in a forming tool or on its own. Reasons for compacting wood are, for example, that it is desirable to increase the hardness of the wood, decrease the moisture content or to obtain a phase in pressure impregnation.
A press of pressure cell type according to that mentioned above is known through SE 452 436. Said patent specification discloses a press plant having a forged, cylindrical press body which requires large, heavy filling blocks to provide a press chamber of an essentially rectangular cross-section. In order to handle the large forces to which the press body is exposed in connection with pressing, the body is wound with steel wire. A press of this type has to be ordered before being manufactured and the complicated work of forging and winding the press body requires several months. It usually takes 15–18 months from order to delivery. The delivery itself is very complicated since, on the one hand, the size of a large press makes road or railway transport difficult and, on the other, the weight amounts to tens of tonnes.
Attempts have been made to manufacture lighter presses having less material in relation to the size than conventional presses have. However, technical problems relating to both manufacturing technique and strength have resulted in these presses only managing a limited working pressure that has been too low with respect to the high pressure which is required in a modern press plant, typically up to 1200 bar.